A 4-year-old girl in Oregon has died from an E.coli infection and a 5-year-old boy in Washington state was in critical condition Wednesday suffering from what was likely the same strain of bacteria, health officials said.

The two children spent time together over the Labor Day weekend, sharing food at an Oregon restaurant and swimming in a pond, their family told health officials in both states.

Bradley Sutton, 5, was listed in critical condition at a Tacoma, Washington hospital on Wednesday after testing positive for E. coli, a spokeswoman at Mary Bridge Children’s Hospital said.

On Monday, his 4-year-old friend, Serena Profitt, died from a form of kidney failure called hemolytic-uremic syndrome, or HUS, which can develop in young children or the elderly after exposure to certain extremely dangerous forms of E.coli, according to the Mayo Clinic.

“We have not yet got the results back from the sample we are testing to identify the specific strain of E.coli that’s involved in this case,” said Tim Prudhel, a spokesman for Lincoln County Health and Human Services.

An investigation into the source of the bacteria, and whether it was foodborne, was also under way.

“It’s a high-priority, active investigation,” Prudhel said.

Possible sources of the E. coli infection would include high-risk foods such as undercooked meat, unpasteurized milk or juices, restaurants at which cases have eaten, exposure to live animals, recreational water, and exposure to child care centers, Oregon Public Health officials said.

Elizabeth Sutton, Bradley’s mother, told KIRO-TV said the two children shared a turkey sandwich at an Oregon restaurant over the Labor Day holiday and swam in a pond together.

Both fell ill days later, complaining of abdominal cramps and had low-grade fevers, she told the station.

Most of the multiple strains of E. coli are harmless or cause relatively brief illness but a few, including O157:H7, can cause severe illness and can lead to HUS, the Mayo Clinic said.

(Reporting by Victoria Cavaliere; Editing by Scott Malone and James Dalgleish)